The viscosity of heavy oil is affected by presence of high molecular weight compounds and their aggregates. Some of these high molecular weight compounds are useful hydrocarbons; however, they cause challenges that impact producing and refining the heavy oil. Efforts to decrease the viscosity of heavy oil involve in-situ precipitation of the high molecular weight compounds. Light, aliphatic hydrocarbons have been used in cyclic solvent injection systems to precipitate some of the high molecular weight compounds. Inhibitors such as polynuclear aromatic hydrocarbons with sulfonic acid functional groups have been added to heavy oil to decrease viscosity of the heavy oil and reduce toluene insoluble material, which is organic and organometallic matter produced in thermal treatment of the heavy oil. Some of the inhibitors can be toxic and, once introduced downhole, can last for an extended period.
Components of heavy oil include paraffins, naphthenes, olefins, resins, asphaltenes, and the like. The chemical structures some of these components include unsaturated hydrocarbon or aromatic portions. Heavy oil also contains heavy metals (e.g., vanadium, nickel, molybdenum, etc.) as well as heteroatoms (e.g., nitrogen, sulfur, and oxygen). The heteroatoms may substitute for carbon atoms in the various heavy oil components. Some of these components are toxic and negatively affect the environment due to, for example, aromatic content.
Asphaltene molecules have been widely reported as having a fused polyaromatic ring system and containing heteroatoms such as sulfur, oxygen, nitrogen, and the like. The heteroatoms may be part of the aromatic ring system or part of other carbocyclic rings, linking groups, or functional groups. Two structural motifs for asphaltene molecules are the so-called continental and archipelago structures. In the continental structure, alkyl chains connect to and branch from a central polyaromatic ring system, which is believed to contain several fused aromatic rings, e.g., 10 or more aromatic rings. In the archipelago structure, multiple polyaromatic ring systems are connected by alkyl chains that may contain a heteroatom, and additional alkyl chains extend freely from the polyaromatic rings. The number of fused aromatic rings in the continental structure can be greater than the number of fused aromatic rings in the archipelago structure.
In addition to the aromatic regions of the asphaltenes, heteroatoms provide the asphaltenes with polar regions, and the terminal alkyl chains provide hydrophobic regions. Asphaltenes also can have polar functional groups such as carbonyl, carboxylic acid, pyrrole, amide, phenol, thiol, etc. Consequently, it is believed that asphaltene molecules aggregate into various micellular structures in oil, with the alkyl chains interacting with the aliphatic oil components and aromatic or polar regions being aligned near the interior of the aggregate or micelle. Precipitation of asphaltenes is widely believed to occur when asphaltenes have a concentration exceeding a critical micelle concentration.
Heavy oil has been characterized as having an API gravity less than 20° with a viscosity at reservoir conditions of lower than 10,000 centipoise, a higher boiling point than lighter ends of crude oil, and a high density compared to lighter oil. The viscosity of heavy oil is one reason why its production has proven difficult, expensive, and time consuming. Furthermore, heavy oil may deposit in the pores of formations, blocking the flow of fluids. Additionally, components of heavy oil, e.g., asphaltenes, can precipitate from a stream of oil and coat boreholes, production tubing, and transport lines, foul processing equipment, or poison catalysts. Despite issues with precipitates from heavy oil, many attempts to upgrade heavy oil still precipitate heavy oil components. Thus, new materials and methods for mobilizing heavy oil would be well received in the art.